Fresh Hydroponic Onions Growing Guide

Growing onions without soil might sound like a novelty, but hydroponic onions can be one of the most rewarding experiences in a home or commercial garden. Over the past few years, I’ve guided several growers through hydroponic setups, and I’ve noticed that onions, especially hydroponic green onions, tend to thrive when they get the right balance of nutrients, light, and water. 

Throughout this article, I will be providing my own experience, some facts, and proven techniques. If you have ever wondered, “Can onions be grown hydroponically?” The answer is yes—and it’s simpler than you would have thought. 

Why Grow Onions Hydroponically? 

While there are traditional ways of growing onions, why should we choose to do it hydroponically? Here are the answers: 

Growing Fast 

One of the more apparent benefits is how fast onions grow as they are being hydroponically cultivated. After initially switching over to onion hydroponics, I recorded up to a 30% increase in the growth rate compared to my soil-cultivated onions. This increased spread, based on one University of Florida study, can mean less time to wait and higher crop turnover per year for hydroponic scallions (green onions). 

Control 

Because you’re taking the nutrients directly to the roots, hydroponics reduces common soil problems like irregular watering or lack of nutrients. This struck me as I noticed hydroponic onion seedlings were uniform and healthy, while my onions in the soil occasionally had a size difference. 

Space Efficiency  

Green onion hydroponics is also preferred by indoor gardeners and city gardeners because of its compact setups. I was able to harvest around 40–50 scallions in four weeks in a small 5-square-foot indoor growing area. With methods like the Kratky system or vertical towers, it is simple to scale up or down based on your space. 

Environmental Benefits 

It’s calculated that hydroponic systems can use 80–90% less water than traditional soil agriculture, in line with USDA water-saving advantages of controlled-environment agriculture. In addition, most hydroponic gardens don’t require chemical pesticides if they’re well-maintained, reducing the chemical load on you and the environment. 

Best Onion Varieties for Hydroponics 

For different types of onions, there are different ways and timelines: 

Green Onions (Scallions)

Scallions are an excellent choice for beginning growers, as they mature more quickly and are less fussy about small mistakes in the nutrient blend. I’ve taught a workshop where a complete beginner had her first scallion crop ready to be harvested in five weeks using a simple hydro system. 

Bulb Onions  

• Short-Day Onions (Texas Super Sweet, Vidalia): Require approximately 10–12 hours of light daily. Wonderful if your environment or climate fails to produce long days. 
• Long-Day Onions (Walla Walla, Yellow Spanish): Need more in the category of 14–16 hours of light to produce fine bulbs. In the event of highly intense grow lights, these may be grown into substantial, delicious bulbs in a hydroponic setup. 

Shallots & Chives

If space is really at a premium or you’d rather have something a little different, shallots and chives work well in hydroponic systems. Because they’re smaller, they’re easy to squeeze in between other plants. 

Tip: Before you buy onion seeds, make sure you check to see if they’re for hydroponic onions grown from seed. There are companies that specialize in seeds specifically designed for soilless systems. 

Setting Up Your Hydroponic Onion Garden  

Choosing a location must be the first step here. Decide if it’s going to be an indoor or outdoor hydroponic garden. 

Indoors vs. Outdoors  

Indoor gardens are easier to control temperature and lighting, but you’ll likely need grow lights. On the other hand, outside gardens need fewer tools to employ, but weather and pests are involved. 

I tried an outdoor NFT system with onions in early spring; a cold snap induced sudden stunted growth, so watch out for weather fluctuations if you live in a changeable climate. 

Materials Needed 

You need seeds or onion sets to germinate. Select varieties characterized as either hydroponic onions or ordinary onions (most will do well if given proper nutrition). Now you need to grow trays and net pots. They can be used for both seed starting and onward growth. 

To keep track of nutrient solution levels and pH, you need a pH meter. You can adjust the level of nutrients for your hydroponic onions to have healthy plants. And if it’s indoors, grow lights are necessary, especially for bulb onions because they require 12–16 hours of light. 

Planting Steps

The germination process starts with planting in rockwool cubes or grow plugs. In one of my experiments, about 85% of onion seeds germinated in 7–10 days with a constant 75°F temperature. After they have firm roots (around 2–3 inches), transplant them. Plant the seedlings in your hydroponic system. Handle them carefully to avoid injuring the roots because they can be extremely fragile. 

Hydroponic Onion Nutrient and Water Requirements 

Optimal nutrient solution is crucial here. For leaf growth, nitrogen is a must if you are a hydroponic green onion specialist. For bulb formation, you will need adequate phosphorus (P) and potassium (K) along with nitrogen (N) to have a balanced NPK. Some hydroponic onion fertilizers are formulated especially for bulb-forming crops. I’ve found brands that encourage slightly more potassium later on. 

Also, the pH level needs to shoot for 6.0-6.8 and a reading of 2.0 mS/cm for EC levels during the establishment period. After the roots are formed, it can be raised to 2.6 EC. So, check and adjust your reservoir frequently. I missed a slowly increasing pH of 7.2 once, and my onions rewarded me with poor growth and a little yellowish hue to the leaves. 

In addition, you should try to keep the water temperature cool between 55–75°F. Higher temperatures can cause root disease; a chiller might be needed if your climate is warm. 

Growing and Keeping Your Hydroponic Onions Healthy

Light exposure is another topic for your hydroponic onions. For example, scallions need 10–12 hours of light, which is generally adequate. Bulb onions should take 12–16 hours for optimal bulb development. My friend, who has a small hydroponic farm, utilizes full-spectrum LED lights, and his Walla Walla onions developed much larger bulbs within 90 days. 

Pruning and spacing are an important part of this process, too. Crowding too many plants can stop air circulation and light penetration. I recommend spacing scallions at least 1–2 inches apart and bulb onions 4–6 inches apart. Remove any curved tips you see to channel energy toward healthy growth. 

Harvesting Hydroponic Onions

It’s important to know when to harvest your onions. For scallions, it takes 4–6 weeks after planting. You can cut them as needed and let them regrow several times. For bulb onions, it usually matures when tops become yellow and fall over. This, depending on the variety, is 80–120 days. 

Proper Techniques 

Cut with sharp scissors or a sterile knife to do as little damage as possible. In harvesting bulb onions, carefully lift them out of the net pot so the roots and bulbs of nearby onions are not damaged. 

Storage Tips  

After growing and harvesting, there are some tips for storage to keep them healthy and fresh. For fresh scallions, you need to keep them in the fridge wrapped in a damp paper towel for a week. For bulb onions, let them dry out (cure) in a dark, cool, airy place for a week or two. Cured onions will keep for 2–3 months. 

Hydroponic Onion Growing Problems and Solutions   

In the hydroponic world, root rots can happen, unfortunately. This often occurs as a result of inadequate oxygen or water temperatures being too high. Algae can grow quickly when excessive light enters the nutrient reservoir. I suggest covering tanks with opaque materials. 

I mistakenly left some onions in a clear reservoir. The algae went crazy within a week’s time, and I ended up having to do an emergency system flush-out. Lesson learned—always keep the reservoir dark! 

Typically, due to a deficiency in light or an unbalanced nutrient, onion bulb development gets lower. A switch to a bloom or fruiting nutrient solution with more potassium typically fixes this problem. 

Fortunately, diseases and pests are less frequent in hydroponics, but also still around are pests like aphids. You should check new plants before adding them to your system. 

Another problem is overcrowding. I’ve seen this with new growers who plant too many seeds in a small net pot. Overcrowding almost always leads to stunted growth. Be sure to thin seedlings early if necessary. 

FAQs

What hydroponic system is best suited for onions?

There is no one “best” system for hydroponic onions, but two of the more widely used ones are Deep Water Culture (DWC) and Nutrient Film Technique (NFT). DWC is simple to use and fine for hydroponic green onions, and NFT is ideal for getting the most space if you have limited space. Both systems provide excellent nutrient and water flow control. 

How do I grow green onions hydroponically from seed? 

Begin by soaking your seeds in water for several hours to stimulate germination, then plant them in rockwool cubes or other seed-starting medium. They like a warm, sunny location, and after seedlings have started, transplant them into your hydroponic system of choice for onions. Keep your nutrient solution at a pH of approximately 5.5–6.5 and provide your system with at least 10–12 hours of light per day.